In recent years, an areal recording density of magnetic disk storage devices (disk drives) has soared at an annual growth rate of about 100%. Meantime, a so-called thermal fluctuation problem is emerging; that is, as the areal recording density becomes higher and higher, magnetically recorded data is erased by surrounding heat. Storage researchers think it is difficult to achieve the areal recording density over 7.75 Gbits per square centimeter on a magnetic drive by conventional longitudinal recording.
Meanwhile, a perpendicular recording method, unlike the longitudinal recording method, has a feature that, as linear recording density increases, a demagnetizing field acting between adjacent bits decreases, and recording magnetization keeps stable. Moreover, a soft magnetic underlayer having a high magnetic permeability formed under a perpendicular recording layer enables magnetic heads to generate a high magnetic field and, consequently, the coercivity of the perpendicular recording layer can be increased. For these reasons, the perpendicular recording method is thought to be one of effective means to overcome the thermal fluctuation limit of the longitudinal recording method.
In the perpendicular recording method, combination of a double-layered perpendicular recording medium that consists of the soft magnetic underlayer and the perpendicular recording layer and a single-pole type head is effective for realizing high-density recording. However, because the double-layered perpendicular recording medium has the soft magnetic layer with a high saturation flux density (Bs), stray magnetic flux generated from magnetic domain walls in the soft magnetic underlayer is observed as spike noise. Moreover, a neighboring track erasure problem that information recorded on neighboring tracks are partially disordered by writing magnetic field and a stray field robustness problem that the stray field in the apparatus concentrates to recording head and the recorded magnetization decays just under the recording head are pointed out.
As means for solving these problems, inserting a hard magnetic pinning layer between the soft magnetic underlayer and the substrate to cause the soft magnetic underlayer to be magnetized in only one direction has been proposed, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei 7-129946 (Document Cited 1) and Japanese Unexamined Patent Application Publication No. Hei 11-191217 (Document Cited 2). Furthermore, a method for suppressing magnetic domain wall motion in the soft magnetic underlayer by an exchange coupling field to an anti-ferromagnetism by, aligning in a single direction, has been proposed, as disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-103553 (Document Cited 3).
[Document Cited 1]
Japanese Unexamined Patent Application Publication No. Hei 7-129946
[Document Cited 2]
Japanese Unexamined Patent Application Publication No. Hei 11-191217
[Document Cited 3]
Japanese Unexamined Patent Application Publication No. Hei 6-103553